1. Field of the Invention
The present invention relates to a vitreous silica crucible and a method of manufacturing the same, and a method of manufacturing a silicon ingot.
2. Description of Related Art
In general, a silicon single crystal is manufactured by melting high-purity polycrystalline silicon in a vitreous silica crucible to obtain silicon melt, dipping an end of a seed crystal to the silicon melt, and pulling the seed crystal while rotating it.
The melting point of silicon is 1410 deg. C., and thus the temperature of silicon melt is kept at a temperature higher than 1410 deg. C. At such temperature, a vitreous silica crucible reacts with silicon melt, and the thickness of the crucible wall gradually decreases. When the thickness of the crucible wall decreases, the strength of the crucible is lowered. This leads to problems such as buckling and sidewall lowering of the crucible.
In order to solve such problem, there is known a technique to promote crystallization of inner surface of a crucible by adding barium into silicon melt (See JP-A-2007-277024). Cristobalite, which is crystalline silica, has higher per-thickness strength than vitreous silica and has lower reactivity with silicon melt than vitreous silica. Therefore, crystallization enhances per-thickness strength and suppresses wall thickness reduction of the crucible.
Furthermore, there is known a technique to provide a layer, on the outside of the crucible, to promote crystallization. When such a layer is provided, the outer layer of the crucible is crystallized and the crucible strength is improved (e.g. JP-A-2000-247778).